Pollen size of Betula in northern Alaska and the interpretation of late Quaternary vegetation records

Abstract

It is commonly asserted in paleoecological studies that shrub birch tends to have smaller pollen than tree birch, but this is poorly documented for Alaskan taxa. We measured freshly dehisced pollen from 55 plants in four northern Alaskan taxa, Betula resinifera (tree birch), Betula glandulosa (shrub birch), Betula nana ssp. exilis (dwarf birch), and Betula resinifera × glandulosa (hybrid). No significant differences existed between mean pollen sizes of any taxa. Betula glandulosa showed significant size variation among sites. Increase in birch-pollen size with time observed in some late Quaternary fossil records from northern Alaska cannot be safely attributed to a change from shrub to tree-dominated vegetation. Alternative explanations for temporal trends in pollen size are (i) evolution of populations, (ii) an environmental effect on pollen size, and (iii) local extinction of a previously widespread taxon. Key words: Alaska, Beringia, Betula, late Quaternary, pollen size.